The present invention relates to a system for a four-wheel drive vehicle in which a power transmission system is changed from a four-wheel drive system to a two-wheel drive system or vice versa in accordance with driving conditions.
A four-wheel drive vehicle which is provided with a transfer clutch for transmitting power of an engine to two auxiliary driving wheels for establishing four-wheel drive is known.
In such a four-wheel drive vehicle, when inspection with respect to structure of the vehicle, engine performance, the emission control system, and other factors take place, main driving wheels are mounted on a free roller and the auxiliary driving wheels are fixed.
U.S. Pat. No. 4,556,134 discloses a four-wheel drive power transmissions system, in which a transfer clutch for the auxiliary driving wheels can be disengaged by operating a switch in order to carry out inspection of the vehicle.
On the other hand, a four-wheel drive vehicle with an automatic transmission has a vehicle speed sensor for controlling the automatic transmission. Since main driving wheels are liable to skid, a sensor is provided for sensing the speed of the auxiliary driving wheels as the vehicle speed. For fail safe, another sensor for sensing the speed of the main driving wheels is provided for use when the sensor for the auxiliary driving wheels fails. The speed of the auxiliary driving wheels is used as vehicle speed to control the transfer clutch. However, since the auxiliary driving wheels must be fixed during the inspection of the vehicle, the vehicle speed can not be detected.